Switched-mode power supply units are generally known. They are clocked transformers for connecting loads to a power network. The clock frequency here is a long way above the frequency of the network, resulting in a much smaller design compared with power network transformers with rectifiers. On the output side switched-mode power supply units supply direct voltages for a wide range of devices.
Switched-mode power supply units generally operate with pulse width modulation (PWM) and regulated constant output voltage. The transferable power here is a function of the design of the switched-mode power supply unit and the deployment conditions. Thermal conditions in particular have to be taken into account here, as the components disposed in a switched-mode power supply unit can only operate within a specific temperature range. The manufacturers of switched-mode power supply units therefore specify a maximum ambient temperature in data sheets for the transferable nominal power. What is known as derating is generally provided for higher ambient temperatures. The manufacturers hereby specify in data sheets by how much it is necessary to reduce the maximum transferable power to prevent component damage at higher ambient temperatures.
Other factors, apart from high ambient temperatures, can also lead to the overheating of switched-mode power supply units. Examples include inadequate heat dissipation or overloading due to short circuits.
Arrangements and methods, with which thermal conditions are monitored and the switched-mode power supply unit is temporarily or totally disconnected, if a limit value is exceeded, are therefore known from the prior art, to protect components from damage in the event of overheating.
JP 7015953-A1 for example describes a switched-mode power supply unit, having a temperature-controlled protective circuit for disconnection in the event of an output-side short circuit. The temperature is measured in proximity to the switching element using a temperature sensor and converted to a digital signal in an analog/digital converter. A timer, to which the digital temperature signal is supplied, is then used to determine the time for disconnection of the switching element as a function of the measured temperature when a short circuit occurs.
JP 2004297886-A1 describes a central arrangement of a temperature sensor in a switched-mode power supply unit, in order to be able to measure the temperature of a plurality of components, such as the switching element, transformer and rectifier, using this one sensor.
The TOPSwitch product family made by the manufacturer Power Integration Inc. has a PWM controller with temperature monitor. Here an analog switching circuit causes the switching element of the PWM controller to trip, as soon as the barrier junction temperature exceeds a predetermined value. Once the temperature drops below a lower threshold value, the switching element starts to switch again and the switched-mode power supply unit operates in normal mode.
Transistors provided for deployment in switched-mode power supply units and having an integrated thermal sensor are also known. With these a thermal sensor chip is disposed in the transistor and connected to its gate and source terminal. In the event of thermal overloading, the thermal sensor chip short circuits the gate and source terminal. A resistor connected in series to the gate terminal causes the trigger voltage to drop below the threshold value and the load switching circuit is disconnected.
The instances of temperature monitoring for switched-mode power supply units known from the prior art therefore relate to protection of the components of the switched-mode power supply units themselves. These are essentially the PWM controller and the switching elements. Thermal overloading then always causes the switched-mode power supply unit to disconnect and no longer to switch, until new operational readiness is recognized when the temperature drops or operators switch the switched-mode power supply unit back on again.